1. Field of the Invention
The invention relates to a projection objective for imaging a pattern arranged in an object surface of the projection objective onto an image surface of the projection objective using ultraviolet radiation.
2. Description of the Related Art
Projection objectives are, for example, employed in projection exposure systems, in particular wafer scanners or wafer steppers, used for fabricating semiconductor devices and other types of microdevices and serve to project patterns on photomasks or reticles, hereinafter referred to generically as “masks” or “reticles,” onto a substrate having a photo-sensitive coating with ultrahigh resolution on a reduced scale.
In order to create even finer structures, it is sought to both increase the image-side numerical aperture NA of the projection objective and employ shorter operating wavelengths, preferably ultraviolet radiation with wavelengths λ<260 nm.
There are very few materials, in particular, synthetic quartz glass (fused silica) and crystalline fluorides such as calcium fluoride, that are sufficiently transparent in that wavelength region available for fabricating the optical elements. The high prices of the materials involved and limited availability of crystalline calcium fluoride in sizes large enough for fabricating large lenses represent problems. Measures that allow reducing the number and sizes of lenses employed and simultaneously contribute to maintaining, or even improving, imaging fidelity are thus desired.
In the case of reducing optical imaging, in particular of projection lithography, the image-side numerical aperture NA is limited by the refractive index of the surrounding medium in image space. This is why in conventional projection systems having a gas in an image space between an exit surface of the projection objective and the substrate the image-side numerical aperture is limited to values NA<1. In immersion lithography the theoretically possible numerical aperture NA is limited by the refractive index of the immersion medium adjacent to the substrate surface. The immersion medium can be a liquid or a solid. Solid immersion is also spoken of in the latter case.
However, for practical reasons NA should not come arbitrarily close to the refractive index of the last medium (i.e. the medium closest to the image surface), since the propagation angles then become very large relative to the optical axis. It has proven to be practical for the image-side numerical aperture not substantially to exceed approximately 95% of the refractive index of the last medium of the image side. For 193 nm, this corresponds to a numerical aperture of NA=1.35 in the case of water (nH2O=1.43) as immersion medium.
With liquids whose refractive index is higher than that of the material of the last lens, or in the case of solid immersion, the material of the last lens element (i.e. the last optical element of the projection objective adjacent to the image) acts as a limitation if the design of the last end surface (exit surface of the projection objective) is to be planar or only weakly curved. The planar design is advantageous, for example, for measuring the distance between wafer and objective, for hydrodynamic behavior of the immersion medium between the wafer to be exposed and the last objective surface, and for their cleaning. The last end surface must be of planar design for solid immersion, in particular, in order to expose the wafer, which is likewise planar.
For deep ultraviolet radiation (DUV, operating wavelength of 248 nm or 193 nm) the materials normally used for the last lens are fused silica (synthetic quartz glass, SiO2) with a refractive index of nSiO2=1.56 or CaF2 with a refractive index of nCaF2=1.50. The synthetic quartz glass material may also be referred to simply as “quartz” in the following. Because of the high radiation load in the last lens elements, at 193 nm calcium fluoride is preferred for the last lens since synthetic quartz glass would be damaged in the long term by the radiation load. This results in a numerical aperture of approximately 1.425 (95% of n=1.5) which can be achieved. If the disadvantage of the radiation damage is accepted, quartz glass still allows numerical apertures of 1.48 (corresponding to approximately 95% of the refractive index of quartz at 193 nm). The relationships are similar at 248 nm.
International patent application PCT/EP2004/014062 filed by the applicant on Dec. 10, 2004 discloses catadioptric projection objectives suitable for immersion lithography at NA>1 comprising a first, refractive objective part for imaging a pattern provided in the object surface into a first intermediate image, a second, catoptric (purely reflective) objective part for imaging the first intermediate image into a second intermediate image, and a third, refractive objective part for imaging the second intermediate image directly onto the image surface. The second objective part includes two concave mirrors having mirror surfaces facing each other, where each concave mirror is positioned optically remote from a pupil surface. The design is rotational symmetric and has one straight optical axis common to all refractive and reflective optical elements (in-line system). Projection objectives of this basic design are disclosed e.g. in U.S. provisional application 60/536,248 filed on Jan. 14, 2004 by the applicant. PCT/EP2004/014062 discloses embodiments having at least one optical element made from high-index material with a refractive index n≧1.6 at the operating wavelength. At 193 nm, sapphire (Al2O3) is used as high-index material for a plano-convex lens forming the last lens of the projection objective. In some embodiments, Cyclohexane is used as an immersion fluid, which has a refractive index n=1.556 similar to that of fused silica (n=1.560) at 193 nm. An embodiment designed for solid immersion (contact projection lithography) has a piano-convex sapphire lens (nsapphire=1.92) which enables NA=1.6 in the disclosed embodiment.
U.S. provisional application 60/544,967 filed by the applicant on Feb. 13, 2004 shows catadioptric projection objectives having similar basic design (as disclosed e.g. in U.S. provisional application 60/536,248 mentioned above) adapted for use with immersion liquids having a refractive index which is larger than the refractive index of the last lens on the image side. The projection objective is designed in such way that the immersion fluid is curved convexly towards the projection objective during operation. The convex curvature of the immersion fluid allows to use larger incidence angles for projection radiation on the interface between the last lens and the immersion fluid, whereby reflection losses at this interface are decreased and higher values for NA are possible, which may be larger than the refractive index of the last lens.
The disclosures of the applications mentioned above are incorporated into the present application by reference.
Various catadioptric in-line projection objectives have been proposed, From an optical point of view, in-line systems may be favorable since optical problems caused by utilizing planar folding mirrors, such as polarization effects, can be avoided. Also, from a manufacturing point of view, in-line systems may be designed such that conventional mounting techniques for optical elements can be utilized, thereby improving mechanical stability of the projection objectives.
The Patent EP 1 069 448 B1 discloses catadioptric projection objectives having two concave mirrors facing each other and an off-axis object and image field. The concave mirrors are part of a first catadioptric objective part imaging the object onto an intermediate image positioned adjacent to a concave mirror. This is the only intermediate image, which is imaged to the image plane by a second, purely refractive objective part. The object as well as the image of the catadioptric imaging system are positioned outside the intermirror space defined by the mirrors facing each other. Similar systems having two concave mirrors, a common straight optical axis and one intermediate image formed by a catadioptric imaging system and positioned besides one of the concave mirrors is disclosed in U.S. patent application US 2002/0024741 A1.
U.S. patent application US 2004/0130806 (corresponding to European patent application EP 1 336 887) discloses catadioptric projection objectives having off-axis object and image field, one common straight optical axis and, in that sequence, a first catadioptric objective part for creating a first intermediate image, a second catadioptric objective part for creating a second intermediate image from the first intermediate image, and a refractive third objective part forming the image from the second intermediate image. Each catadioptric system has two concave mirrors facing each other. The intermediate images lie outside the intermirror spaces defined by the concave mirrors.
Japanese patent application JP 2003114387 A and international patent application WO 01/55767 A disclose catadioptric projection objectives with off-axis object and image field having one common straight optical axis, a first catadioptric objective part for forming an intermediate image and a second catadioptric objective part for imaging the intermediate image onto the image plane of this system. Concave and convex mirrors are used in combination.
U.S. patent application US 2003/0234992 A1 discloses catadioptric projection objectives with off-axis object and image field having one common straight optical axis, a first catadioptric objective part for forming an intermediate image, and a second catadioptric objective part for imaging the intermediate image onto the image plane. In each catadioptric objective part concave and convex mirrors are used in combination with one single lens.
International patent application WO 2004/107011 A1 discloses various catadioptric projection objectives with off-axis object field and image field having one common straight optical axis designed for immersion lithography using pure water (H2O) as immersion liquid and an arc shaped effective object field having a field center far away from the optical axis. The projection objectives include various types of mirror groups having two, four or six curved mirrors. Embodiments with two to four intermediate images are disclosed. Silicon dioxide (SiO2) and/or Calcium fluoride (CaF2) are used as lens material. Image-side numerical apertures up to NA=1.2 are obtained.
From a practical point of view, the maximum image-side numerical aperture which can be achieved is only one of a number of design goals for which an optimum compromise must be found depending on limiting conditions imposed for a specific application. In view of the limited availability of optical materials, such as Calcium fluoride, the maximum size (diameter) of lenses needed for a design may be a limiting factor. Typically, the maximum lens diameter necessary in a design is increased as the image-side numerical aperture increases. Specifically, very large diameters are needed in an aperture-defining lens group arranged on the image side between and image-side pupil surface of the projection objective and the image surface. The aperture-defining lens group is basically designed for converging radiation coming from the image-side pupil surface towards the image surface to define the image-side numerical aperture. With other words, the aperture-defining lens group performs the pupil imaging. Also, the focal length of the aperture-defining lens group responsible for the pupil imaging tends to decrease as the image-side numerical aperture is increased. Shortening the focal length of the aperture-defining lens group, however, makes it more difficult to provide sufficient correcting means for the pupil imaging since the number of optical surfaces within the aperture-defining lens group is limited.